Marine species have developed some pretty unique methods of mating, some can reproduce asexually not even requiring a partner, while other will release their gametes (sex cells) into the water column and hope for the best. Within the phylum Cnidaria (containing jellyfish, corals, and anemones), a diverse range of mating strategies exists, all of which have evolved as a balance of parental energy and investment and the number of surviving offspring. Most species in this phylum use external sexual fertilization, usually done by broadcast, or mass spawning. In this scenario, males receiving environmental or pheromone cues will release clouds of free-swimming sperm in hopes that they will find their way to the eggs of the females and fertilization will occur (Fig. 1). But in the class Cubozoa, a much more complex and unique reproductive method has evolved.

Fig. 1: A mass spawning event for corals can be a sight to see. For corals, they just release their gametes and hope for the best! (scubadiving.com)

The class Cubozoa includes the infamous box jellyfish, which is regarded as one of the most venomous creatures on earth and is known for its small size and deadly stings. Some species within Cubozoa have pretty elaborate behaviors, likely a result of their sensory ability. These species have also evolved a form of internal fertilization. In this scenario, the male jellyfish will use a tentacle to deliver a sperm package, called a spermatozeugmatas, to the female jellyfish who can then store the package and fertilize the eggs. The box jellyfish species Copula sivickisi (Fig. 2) exhibits this behavior. Females of this species will lay down strands of embryos 2-3 days after mating, a behavior not observed in any other jellyfish.

Fig. 2: A male C. sivickisi. The bright orange globs are the gonads.

With so little known about the reproduction in this species, researcher Dr. Anders Garm and colleagues at the University of Copenhagen decided to take a deeper look. What they found was quite extraordinary.

The Study:

In 2012, mature males and females of C. sivickisi were collected off the coast of Japan. Keep in mind these jellyfish are tiny, only growing to about ½ inch in diameter. They were preserved in the lab and their reproductive structures were investigated using microscopy. Observed behaviors (seen in the field) were combined with with the information gained by investigating structures up close (via microscopy) to better understand jellyfish mating.

Fig. 3: A male and female swimming together while mating.

When a male and female C. sivickisi mate, the male grabs hold of the female and they begin to swim together (Fig. 3). During this time, the male uses one of its free tentacles to transfer a sperm package (about 500um in width, or roughly one-third the size of the head of a pin) to the female (Fig. 4). Researchers discovered that the female actually stores the sperm package in her stomach. Here’s where things get interesting. It was found that the male gonads contained cells called cnidocytes on the surface layer. Cnidocytes are stinging cells, the cells that help capture prey and the ones used for defense, the same cells that result in human deaths! So yes, male gonads contain stinging cells and these cells are also present in the sperm packages delivered to the female. But before you start imagining what that would be like, it was found that although these cells are present, they appear have lost their stinging function. As a female jellyfish you might be breathing a sigh of relief, but males should be on alert. Researchers also found high concentrations of these stinging cells in female gonads!

Fig. 4: The spermatozeugmatas, or sperm package. The arrows point to the cnidocytes, or stinging cells.

Fortunately, for both parties involved in the mating process, stinging cells do not hinder reproduction. Researchers found that males deliver the sperm package to the females where it is then stored in the females stomach before being brought to the gonads for fertilization. Most internal fertilizations occurs with sperm actively seeking the eggs.. In contrast, female C. sivickisi start to “eat” the sperm, using enzymes to partially digest the sperm package and the sperm inside. This process releases the nuclei from the sperm and transfers them to the gonads to fertilize the eggs (Fig. 5). From there, the fertilized eggs are laid on a surface and 2-3 days later, larvae will hatch.

Fig. 5: A fertilized female C. sivickisi. The gonads have become enlarged and appear brownish in color.

The Significance:

So why the need for stinging cells if they are not being used for defense or prey capture? The researchers suggest that the non-functional stinging cells that were transferred from the male gonads to their sperm packages are useful in attaching themselves to the females and securing fertilization. The stinging cells found in the female remain active and are later transferred to the fertilized embryos. In this manner stinging cells are ultimately used as a protective mechanism or deterrent for those looking to eat the eggs (Fig. 6).

Fig. 6: The fertilized embryos (E) are surrounded by stinging cells, as indicated by the arrows in the inset.

This study sheds light onto poorly understood, and seemingly otherworldly, reproductive strategies. For C. sivickisi, evolving internal fertilization and the use of stinging cells is an adaptation for increased larval survival, via parental investment. Those marine species employing mass spawning see incredibly low survival rates of larvae. While C. sivickisi is limited in terms of the number of embryos it can produce, the survival rate is going to be much higher due to stinging cell protection.

Here we are, in 2015, and still discovering completely new and bizarre adaptations of life on earth. Much of the ocean still remains a mystery, get out there and explore!

I am currently a postdoc at Keck Sciences, Claremont McKenna College. I work with Dr. Sarah Gilman, measuring and modeling energy budgets in intertidal species. I am a climate scientist and marine community ecologist and my PhD (University of Rhode Island) focused on how ocean acidification and eutrophication, alters coastal trophic interactions and species assemblages.